This invention relates to optical systems for use where an optical image is to be formed of an object surface which may not be normal to the optical axis of the imaging system, enabling the image to be well focused over the full image area, on a surface which is normal to the optical axis of the imaging system, and the invention relates more generally to optical systems in which the tilt of an in-focus image surface, with respect to an axis of the system is to be altered or adjusted.
In many optical imaging applications it is necessary to form an image of a surface which is not normal to the axis of the optics. In photography, for example, a camera is frequently held with its lens axis horizontal and a picture is taken of a horizontal surface such as a road or a table top. In a camera, as in most image forming instruments, the surface on which the final image is required, in this case the film plane, is normal to the optical axis. However, if an object surface is tilted with respect of the normal to the optical axis, then the corresponding image surface at which an in-focus image is formed is also tilted with respect to the normal to the optical axis. According to the Scheimpflug rule, the planes including the axial portions of object surface and image surface respectively, if extended, will intersect at the plane including the imaging lens. It follows that, when an image of a nonnormal surface is formed, the image is generally out-of-focus on the required image surface, except for a narrow band where the tilted, in-focus image plane intersects with the required image surface.
In practice, optical systems have a certain depth of focus which, in relation to an image surface, may be defined as that volume which is bounded by two surfaces, one on each side of the in-focus surface, within which the extent of image defocus is acceptable. In most photographic and other image forming optical systems, it is generally possible to arrange that in-focus image surfaces, albeit tilted with respect to the film plane or other surface on which an image is required, lie within the depth of focus of the systems.
In some cases, however, it is desirable to optimise focus over a large image area by compensating for the in-focus image surface tilt due to a non-normal object surface. It is common practice in such cases to introduce a tilt, either of the surface on which a final image is required, for example the film plane, or of the lens. The tilt in either case is calculated, using the Scheimpflug rule, so that the in-focus image surface coincides with the surface on which the image is required.
We have so far considered optical systems, such as cameras, in which a final image is preferably to be formed on a surface orthogonal to the optical axis. In such cases it is desirable to compensate for tilt of object surfaces. In some optical systems it is necessary to form an image on a surface which is not orthogonal to the optical axis of the projection optics, although it will generally be convenient for the corresponding object surface to be orthogonal or nearly orthogonal to the optical axis. In such optical systems, it is generally desirable to produce tilt in the in-focus image surface to match the tilt of the surface upon which the image is to be projected.
Any optical arrangement which may be used to compensate for tilt in an object surface may alternatively be used to produce tilt in an in-focus image surface. The positions of object surface and in-focus image surface may simply be reversed. Thus, in a slide projector system in which an image is required to be projected onto a screen not orthogonal to the radiation propagation direction, the focus of the image on the screen may be optimised by tilting the projector lens by an angle calculated according to the Scheimpflug rule.
In some optical systems, very large tilts appear in in-focus image surfaces, so that it becomes difficult, using available components, to compensate for the tilt by the classical method of tilting elements in the optical system. The present invention may be used in such optical systems.
Also, in some optical systems, it is necessary to accommodate very large tilts in the surface on which a final image is required so that it is difficult, using available components, to produce the required tilt by the classical method. The present invention may be used in optical systems in which either a large tilt of an object surface must be compensated or a large tilt must be produced in an in-focus image surface. It is to be particularly noted that the same devices may be used either for tilt compensation or for tilt production.
The invention has a specific use in visual simulation systems, such as are used for example in ground-based, flight simulating apparatus. In such systems, it is common practice to use a model of the terrain to be flown over in simulated flight and to relay an image of the terrain to the trainee crew via a closed circuit television system. An image of the model is first formed onto a television camera target by an optical arrangement known as a probe. The present invention is applied in such probe optics. The entrance pupil of the probe optics is manoeuvred above the model terrain so that the picture relayed to the trainee crew is continuously in the required perspective. To achieve correct perspective, the separation of the probe entrance pupil from the model must be proportional to the simulated aircraft height, in the proportion of the model to real terrain scale ratio.
In a practical flight simulator system, a minimum separation between the probe entrance pupil and the model surface, simulating an aircraft position on a runway, may be only about 2 mm. It is necessary that the entrance pupil diameter of a practical probe should be at least about 0.5 mm in order for the probe to collect sufficient light, and also in order to give an acceptable diffraction limit on probe optical resolution. Since it is difficult to design well-corrected lenses with high numerical apertures, that is large ratio of pupil size to focal length, the focal length of probe optics can in practice be no less than a few millimeters, a reasonable minimum being about 3 mm.
Model surfaces may be regarded as flat and, with a simulated aircraft positioned on a runway, the probe optical axis is effectively parallel to the model surface in most simulator systems. Applying the Scheimpflug rule, and assuming the minimum entrance pupil to model separation and the focal length estimated above, the calculated tilt of the primary in-focus image plane with respect to the normal to the optical axis, is 56.degree..
In a simulator system, an image plane tilt of this order cannot readily be compensated either by tilting the television camera target with respect to the probe optics, or by tilting a lens in the probe optics.
Tilting of the target surface is not mechanically feasible in simulator systems, and would introduce excessive image distortion.
Probe systems have been constructed in which tiltable lenses are used, according to known methods, to provide a measure of compensation for tilt of the primary in-focus image plane. To be effective in compensating for primary image tilt, a tiltable lens must have some dioptric power, that is, power to converge or diverge light beams, and it must be set in a position in the optical system where it has an appreciable converging or diverging effect on beams associated with field-of-view image points, that is, it must not be a field lens set at an image plane. Typically, a single tiltable lens may follow the objective lens of a probe, collimating light received from the primary image formed by the objective. As the primary image tilts, the tiltable lens is tilted through the same angle, so that the primary image remains in the focal plane of the tiltable lens. Thus, light from the whole primary image area is always collimated.
More complex probe systems have been constructed in which more than one lens is tilted, the function of compensating for primary image tilt being effectively divided between the tilting lenses.
In optical systems in which lenses are tilted to compensate tilt of object or primary image surfaces, including complex systems such as simulator probes, it has been common practice for each tilted lens to be rotated about a tilt axis in the region of the lens itself.
Commonly a lens is tilted about a nodal point, in order to avoid movement of the transmitted image due to the lens rotation. In existing systems, tiltable lenses having large field angles but only moderate numerical apertures are used. It is therefore necessary for each tilt lens to be rotated about a tilt axis not far removed from the lens entrance pupil, since large lateral movement of the lens pupil would cause unacceptable vignetting. In no known tilt lens system for compensating primary image tilt, is a tiltable lens rotated about an axis of rotation in or near a neighbouring image plane.
Since, in existing systems, each tiltable lens is tilted about an axis in the region of the lens itself, as a lens is tilted through an angle .theta., the centre of the image to be relayed by the lens moves to a part of the angular field of the lens which is approximately .theta. off the lens axis. To preserve good resolution in the relayed image, the tiltable lens must be corrected for image aberrations over a semifield angle approximately equal to, or somewhat in excess of, the maximum angle .theta. through which the lens is to be tilted. Where comparatively small lens tilt angles are required, as in most photographic applications, the off-axis correction of tilted lenses is generally adequate. However, where large lens tilt angle are required, as in simulator probes, it has been found difficult to provide adequate correction for optical aberrations of the tiltable lenses over their necessarily large field angles, even when more than one tiltable lens is employed to divide the tilt angle to be introduced at each. Thus, existing simulator probes have not provided well-resolved images for the smallest required separations of the probe entrance pupil from the model.
The invention may also be used in visual flight simulation systems as described above, but in which the probe is replaced by a laser beam scanning system which projects a moving beam on to the model terrain and in which laser light reflected from the model terrain is collected on photo-detectors. The photo-detector output signals provide a video signal similar to that which would be provided by a television camera in a probe. In order for the video signal to provide a picture in correct perspective, it is necessary for the exit pupil of the laser beam scanner, that is, the aperture from which the beam is scanned, to be manoeuvred above the model terrain in the same way as the entrance pupil of a probe. The separation of the exit pupil from the model surface must be proportional to the simulated aircraft height in the proportion of model to real terrain scale ratio.
In a practical system, the minimum separation of the laser scanner exit pupil from the model surface may be only about 2 mm. The laser beam width at the exit pupil must be at least about 0.5 mm for at least part of the scan, in order to give an acceptable diffraction limit on the angular resolution of the scan. It is desirable that the laser beam should be continuously in focus on the model surface, even though the surface is at an acute angle to the radiation propagation direction. Beam deflecting devices such as may be used in the laser beam scanning system cannot readily be made to alter the beam convergence or divergence angle as the beam is deflected. If the device scans the beam so that it is in focus in a line or plane, then the line or plane must generally be substantially orthogonal to the radiation propagation direction.
The laser beam scanning system must therefore preferably include an optical system capable of producing a tilt, with respect to the orthogonal to the optical axis or radiation propagation direction, in the surface on which the laser beam reaches its focus. The optical system will be set between the scanning device and the exit pupil. The system will receive light from a real or virtual "object" surface orthogonal to the beam direction, on which the beam is in focus, and form an image of this object surface on an image surface which is generally tilted with respect to the orthogonal to the beam direction.
The problem of producing image surface tilt in a laser beam scanning system is essentially similar to the problem of compensating for object surface tilt in a probe. The same optical systems may in principle be used in the two cases, with object and image surfaces being interchanged in the two cases. The final lens in the laser beam scanning system, which takes the place of the front lens of a probe, may, with a 0.5 mm external pupil, have a minimum focal length of about 3 mm. When the pupil is only 2 mm from the model surface, if the laser beam is to be focussed on the model surface, then the laser beam must also be in focus as it scans along a line or in a plane tilted at about 56.degree. to the back focal plane of the final lens. This is the tilt angle to be produced by a tilt compensation optical system.
One or more tilted relay lenses may be employed in a laser beam scanning system, set between the beam deflecting device and the final lens before the exit pupil. But as in the case of the probe, a tilt angle of order 56.degree. cannot readily be dealt with using available lenses as tilting lenses in a conventional configuration.
One object of the present invention is to provide means for correcting large tilts of an in-focus image surface, so that a final image may be formed on a surface substantially normal to the radiation propagation direction, which is in good focus over the whole image area.
Another object of the invention is to provide means for producing tilt in an in-focus image plane, for example, as required in a laser beam scanning system.